Objects with metal surfaces are frequently joined to objects with plastic surfaces using double-sidedly adhesive 2D elements such as, for instance, adhesive labels, adhesive tapes, adhesive sheets or the like. In order to be able to join different materials firmly to one another it is necessary for the two faces of the 2D element each to have different adhesives, tailored to the respective substrate. Thus an adhesive which ensures a stable adhesive bond on a metal surface is generally of limited suitability at best for bonding to plastics, and vice versa.
Where the faces to be joined are relatively large, extensive 2D elements can be employed which are furnished on both sides with pressure-sensitive adhesives. Because of the large bonding area, the resultant fixing and fastening of the metal components to the plastic elements are sufficiently stable.
Where, however, smaller parts are to be joined to one another, in the field for example of electronics for entertainment and communication, the bond strength which is achievable with pressure-sensitive adhesive systems is often not enough to ensure a stable join. In the case of systems of this kind, therefore, heat-activatedly bonding adhesives are employed, which are bonded while hot and, after cooling, provide a mechanically robust join.
Heat-activatedly bonding adhesives can be classified in principle in two categories: thermoplastic heat-activatedly bonding adhesives, and reactive heat-activatedly bonding adhesives.
Thermoplastic adhesives are based on polymers which on heating undergo reversible softening and solidify again in the course of cooling. A disadvantage of this is that when pressure is applied to thermoplastic adhesives of this kind they may exhibit an adverse flow behaviour (known as “oozing”). The change in shape of the adhesive under pressure makes it disadvantageous to use thermoplastic heat-activatedly bonding adhesives to bond fine structures, since in such situations it is possible for the adhesive to exit from the thin bondline.
Reactive heat-activatedly bonding adhesives, in contrast, comprise elastic components and reactive components. The latter are the resins known as reactive resins, in which heating initiates a process of crosslinking which after the end of the crosslinking reaction ensures a durable, stable bond even under pressure. Of particular interest as elastic components are synthetic nitrile rubbers, which on account of their high flow viscosity give the heat-activatedly bonding adhesive a dimensional stability which is particularly high even under pressure.
This kind of low flow capacity with high dimensional stability, however, may also have disadvantages associated with it, since at low temperatures the heat-activatedly bonding adhesive very rapidly solidifies and becomes brittle. The consequence of this is that, at low temperatures, the adhesive bond is mechanically sensitive towards external influence and may therefore part even on low-intensity impacts. Such sensitivity to impact (shock sensitivity) at low temperatures is particularly undesirable for the adhesive bonding of components in portable equipment, since such equipment is used outdoors as well, where it may be exposed both to high and to low temperatures.
Impact sensitivity is particularly problematic for joins between metal substrates and plastic surfaces, since in the event of impact the plastic is able to absorb some of the energy but the metal frequently does not deform, and so the greatest part of the impact energy must be accommodated by the heat-activatedly bondable 2D element.
A further factor is that the stability of a heat-activatedly bondable 2D element with two different adhesives—one adhesive for bonding to a metallic substrate and another for bonding to a plastic surface—is in any case highly dependent on the prevailing ambient temperature. This is attributable to the difference in the anchoring of the two adhesives to one another or to a common carrier, and is attributable in particular to the temperature dependencies of the thermal expansion behaviour, of the viscosities or of the structure on the temperature, which are different for the two adhesives. Since, therefore, at low temperatures in particular, the anchoring faces of these 2D elements have only a low mechanical load-bearing capacity, these anchoring faces are particularly susceptible to impacts, and so the 2D element splits particularly easily at these points.
It is an object of the present invention, therefore, to provide a heat-activatedly bondable 2D element having a first adhesive and a second adhesive that eliminates these disadvantages and that makes it possible in particular over a broad temperature range to achieve impact-insensitive and stable adhesive bonding of metal substrates to plastic substrates, and also a high dimensional stability. The intention in particular is to provide a heat-activatedly bondable 2D element which is insensitive to impact at −20° C. and offers a high strength of bonding to plastic surfaces and metal surfaces in a temperature range from −20° C. to +50° C.